July 14, 2022

Clear nylon injection molding problems

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Product details: This is a new product, has not completed sign off yet. The mold has not produced consistently good product, and this is the first time we are using this particular material.

Material: PA12 -grilamid TR55 – clear nylon, has a very low viscosity (always get this confused anyway it doesn’t flow very easy).

Mold design: Single cavity, with a cold runner injecting into a threaded section in the center of the bowl, the threaded section is created by an insert which is manually placed into the mold by an operator, the material has to flow over the threaded section causing a lot of turbulence. There is no cold well, and the gate type is a fan type gate.

Process: I have tried different injection speeds and pressures with faster speeds and higher pressures been the most successful (it hides the defect better but does not eliminate it).
The material comes in pre-dried bags and is stored in a de-humidifier whilst processing (therefore do not think it is moisture) I have tried the full range of temperatures recommended for this grade of material finding that Zone 1 to 4 are near the upper to mid temps and the nozzle as low as I can go without freezing off. There is minimal suck back and speed is kept to a minimum. I have tried with and without sprue break and neither seems to make a difference however I prefer without. Screw speeds and back pressure are set to recommendations but I have tried increasing and decreasing, with little to no improvement. The cycle time/dwell time is bordering on being too long. Basically I have tried everything I can think of.

Actual problem: the part presents with silver type streaks that emanate from the injection area they sometimes radiate all the way around like a bicycles spokes or are only in one area but more pronounced. If I run the part and keep the cycle consistent I can almost eliminate the issue however as soon as an operator takes over the problem will get worse within a couple of minutes.

Any suggestion is welcomed.

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